A thin digital still camera such as a card type camera is fabricated year after year, and reduction in size of an imaging unit is demanded. In addition, reduction in size of the imaging unit is also demanded in a mobile phone in order to reduce the thickness of the terminal itself and to secure a space for a lot of functions to be mounted. Therefore, demand for further reduction of the size of the imaging lens mounted on the imaging unit is increasing.
In addition, together with the size reduction of the imaging device such as a CCD and a CMOS, the number of pixels is increased by microfabrication of the pixel pitch of the imaging device. In accordance therewith, high performance is demanded for the imaging lens used in the imaging unit.
High resolution is demanded for the imaging lens used in such an imaging device with high definition; however, the resolution is limited by an F value. Sufficient performance is not obtainable by the F value of about 2.8 because a bright lens with a small F value provides high resolution. Accordingly, an imaging lens with the F value of about 2 that is suitable to the small imaging device with a large number of pixels and high definition is demanded. As the imaging lens for such a purpose, an imaging lens configured of six lenses that allows for increase in aperture ratio and improvement in performance as compared with an imaging lens configured of four or five lenses has been proposed (refer to PTLs 1 and 2).
For example, the imaging lens configured of six lenses described in PTL 1 includes, in order from object side toward image plane side, a first lens having positive refractive power, a second lens having negative refractive power, a third lens, a fourth lens having negative refractive power, a fifth lens having positive refractive power, and a sixth lens. The first lens has a surface with a positive radius of curvature on the object side, the second lens has a surface with a positive radius of curvature on the image plane side, and the fifth lens has a surface with a negative radius of curvature on each of the object side and the image plane side. Each of the third lens to the sixth lens has refractive power lower than the refractive power of each of the first lens and the second lens.